Add support for static variables in GDScript

Which allows editable data associated with a particular class instead of
the instance. Scripts with static variables are kept in memory
indefinitely unless the `@static_unload` annotation is used or the
`static_unload()` method is called on the GDScript.

If the custom function `_static_init()` exists it will be called when
the class is loaded, after the static variables are set.

modules/gdscript/tests/scripts/runtime/features/static_variables.gd

@@ -0,0 +1,56 @@
+@static_unload
+
+static var perm := 0
+
+static var prop := "Hello!":
+	get: return prop + " suffix"
+	set(value): prop = "prefix " + str(value)
+
+static func get_data():
+	return "data"
+
+static var data = get_data()
+
+class Inner:
+	static var prop := "inner"
+	static func _static_init() -> void:
+		prints("Inner._static_init", prop)
+
+	class InnerInner:
+		static var prop := "inner inner"
+		static func _static_init() -> void:
+			prints("InnerInner._static_init", prop)
+
+func test():
+	prints("data:", data)
+
+	prints("perm:", perm)
+	prints("prop:", prop)
+
+	perm = 1
+	prop = "World!"
+
+	prints("perm:", perm)
+	prints("prop:", prop)
+
+	print("other.perm:", StaticVariablesOther.perm)
+	print("other.prop:", StaticVariablesOther.prop)
+
+	StaticVariablesOther.perm = 2
+	StaticVariablesOther.prop = "foo"
+
+	print("other.perm:", StaticVariablesOther.perm)
+	print("other.prop:", StaticVariablesOther.prop)
+
+	@warning_ignore("unsafe_method_access")
+	var path = get_script().get_path().get_base_dir()
+	var other = load(path  + "/static_variables_load.gd")
+
+	print("load.perm:", other.perm)
+	print("load.prop:", other.prop)
+
+	other.perm = 3
+	other.prop = "bar"
+
+	print("load.perm:", other.perm)
+	print("load.prop:", other.prop)